The present invention relates to polymeric film compositions useful in preparing separator membranes for electrolytic cells. For example, such a film material may be used in a rechargeable battery cell as an intermediate separator element containing an electrolyte solution through which ions from a source electrode material move between cell electrodes during the charge/discharge cycles of the cell. The invention is particularly useful for such cells in which the ion source is lithium, a lithium compound, or a material capable of intercalating lithium ions. There, the separator membrane comprises a polymeric matrix which is ionically conductive by virtue of, for example, the incorporation of an organic solution of a dissociable lithium salt which provides ionic mobility.
Early rechargeable lithium cells utilized lithium metal electrodes as the ion source in conjunction with positive electrodes comprising compounds capable of intercalating the lithium ions within their structure during discharge of the cell. Such cells relied, for the most part, on separator. structures or membranes which physically contained a measure of. fluid electrolyte, usually in the form of a solution of a lithium compound, and which also provided a means for preventing destructive contact between the electrodes of the cell. Sheets or membranes ranging from glass fiber filter paper or cloth to microporous polyolefin film or nonwoven fabric have been saturated with solutions of a lithium compound, such as LiClO.sub.4, LiPF.sub.6, or LiBF.sub.4, in an organic solvent, e.g., propylene carbonate, diethoxyethane, or dimethyl carbonate, to form such electrolyte/separator elements. The fluid electrolyte bridge thus established between the electrodes has effectively provided the necessary Li.sup.+ ion mobility at conductivities in the range of about 10.sup.-3 S/cm.
Although serving well in this role of ion conductor, these separator elements unfortunately comprise sufficiently large solution-containing voids, or pores, that continuous avenues may be established between the electrodes, thereby enabling lithium dendrite formation during charging cycles which eventually leads to internal cell short-circuiting. Some success has been achieved in combatting this problem through the use of lithium-ion cells in which both electrodes comprise intercalation materials, such as lithiated manganese oxide and carbon (U.S. Pat. No. 5,196,279), thereby eliminating the lithium metal which promotes the deleterious dendrite growth. Although providing efficient power sources, these lithium-ion cells do not readily attain the capacity provided by lithium metal electrodes.
Another approach to controlling the dendrite problem has been the use of continuous films or bodies of polymeric materials which provide little or no continuous free path of low viscosity fluid in which the lithium dendrite may propagate. These materials may comprise polymers, e.g., poly(alkene oxide), which are enhanced in ionic conductivity by the incorporation of a salt, typically a lithium salt such as LiClO.sub.4, LiPF.sub.6, or the like. A range of practical ionic conductivity, i.e, over about 10.sup.-5 to 10.sup.-3 S/cm, was only attainable with these polymer compositions at ambient conditions well above room temperature, however. Some improvement in the conductivity of the more popular poly(ethylene oxide) compositions has been reported to have been achieved by radiation-induced cross-linking (U.S. Pat. No. 5,009,970) or by meticulous blending with exotic ion-solvating polymer compositions (U.S. Pat. No. 5,041,346). Each of these attempts achieved limited success due to attendant expense and restricted implementation in commercial practice.
Some earlier examinations of poly(vinylidene fluoride) polymers and related fluorocarbon copolymers with trifluoroethylene or tetrafluoroethylene revealed enhancement of ionic conductivity by a simpler incorporation of lithium salts and organic solvents which are compatible with both the polymer and salt components. This work by Tsuchida et al. (Electrochimica Acta, Vol. 28 (1983), No. 5, pp. 591-595 and No. 6, pp. 833-837) indicated, however, that the preferred poly(vinylidene fluoride) compositions were capable of exhibiting ionic conductivity above about 10.sup.-5 S/cm only at elevated temperatures, reportedly due to the inability of the composition to remain homogeneous, i.e., free of deleterious salt and polymer crystallites, at or below room temperature. Such limitations apparently led to the abandonment of attempts to implement these compositions in practical applications, such as rechargeable battery cells.
The present invention provides a means for avoiding the disadvantages of prior electrolytic cell compositions and constructions by enabling the ready and economical preparation of strong, flexible polymeric electrolytic cell membranes which will readily retain electrolyte salt solutions and remain functional over a range extending well below room temperature.